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Topic: Let's discuss Piston Speeds (Read 1894 times)

Okay, it's rainy and I think we need to discuss the subject of piston speeds, and what that means in relation to our Bullets.There's been some discussion lately about pistons, and why certain piston characteristics are important, including the mass of the piston and pin.

So here we go.

A long time ago, there was this automotive engineer by the name of Laurence Pomeroy.He came up with a standard called the "Pomeroy Dictum". The Pomeroy Dictum states that for good longevity, the piston speeds in an internal combustion engine should remain under 2500 feet per minute.Okay that's a relatively older standard, but it's still relevant, and it was an accepted premise when the Bullet engine was originally developed in the 1940s/50s.

Why is this important?An engine has reciprocating parts that move very quickly, and this causes stress to them. If speeds of these reciprocating parts exceeds what they are capable of withstanding, they either distort(stretch or compress or twist) or they break. Sometimes this can lead to metal fatigue, and eventual failure, even if they don't break right away.

What's the part at biggest risk? The part at biggest risk in our Bullets is the connecting rod, although the big end bearing and the main bearings are also somewhat at risk. And the crankshaft itself is at lesser risk, but if it is flawed or defective in some way, higher piston speeds and also rpms can bring it into failure more quickly.

So, what do we do about it?Well, for users of the stock rod, and especially when changing to a larger and heavier piston which increases moving mass, we need to observe rpm limits, so that we stay in a relatively safe zone for the parts to handle. If we've increased piston mass, but haven't done anything to improve rod strength, then we must observe a lower rpm limit for safety. But this often goes contrary to what the user wants to do. Often, the user wants to put in a bigger heavier piston for performance purposes, and also wants to rev higher for performance purposes. This can lead to a problem. Not always, and not necessarily right away, but the issue does loom. If we use a stronger connecting rod and better bearings, we can hope to avoid the problems, but it's not guaranteed to do so. But, it is very likely to allow more revs with heavier piston mass if you do make the improvements to the other parts involved.

What are the piston speeds in the Bullet?Well, being a long stroke engine, with a 90mm stroke, they can get pretty fast at lower rpms than a shorter stroke engine. And this is why our redlines are not as high as Japanese engines, or other short stroke designs.

For example, with our Bullet if we wanted to stay within the Pomeroy Dictum2500fpm with a 90mm stroke = about 4250 rpms. Not very high, is it?So, we already exceed Pomeroy in stock form with our engine, because we redline around 5000 rpm.How fast is that?5000rpm with a 90mm stroke = about 2950 fpm, or 20% over Pomeroy. Getting a bit touchy, but our engine can handle it.How about 5500 rpm?5500rpm with 90mm stroke = about 3250 fpm. that's 30% over Pomeroy.6000rpm?6000rpm with 90mm stroke = about 3550 fpm, or 40% over Pomeroy.

Now, some comparisons.Many street cars today, still remain under Pomeroy's Dictum, and perform very well for a long time with piston speeds less than 2500 fpm.But what about hi performance cars?Well a Honda S2000 which is a fast production car with overhead cams redlines with a piston speed of 3850 fpm. This is a very modern engine that has all the latest technology in it. And it's not an awful lot higher than if we try to do 6000 rpm with our old antique design.An extreme case might be the Ferrari Formula One car. It does 18000 rpms, and has a piston speed of 4890 fpm, almost double the Pomeroy Dictum. But this represents the highest level of design and construction of about anything on the planet. That's not where we are with our engines.

So, where does this all lead us?Well, we need to understand just how much we are exceeding the Pomeroy Dictum with our revving. And we know that we can use an everyday limit of around 4250 rpm for daily driving, and have good longevity with our Bullets. And this has been proven over the years, and holds true to form for Pomeroy.When we want to go fast, we can rev to 5000 rpm, and exceed Pomeroy by 20%, and this is okay on an occasional basis, and maybe even frequent basis. But it will yield some additional stress, and may shorten engine life some. But, the engineers apparently felt that it would be okay for a redline of that amount. I don't think they had it in mind that people would run around at 5000 rpm all day, or even do it at all every day.The rest is a "call shot" on the part of the owner, as to how far above Pomeroy's Dictum or the designed limits that he wants to risk with his own bike. 5500rpm is an additional 10% over what the Enfield engineers envisioned as a redline. I think it's do-able. I don't think it's prudent to "live" up there in those rpms, but I think it can be touched when you want to ride hard on an occasional basis, and accept the risk.6000 rpm in a Bullet is 20% higher than the Enfield engineers designed for, and is pretty much pushing the limits. It's well over Pomeroy, and well over what Enfield had in mind when they made these parts. So, that is gettting pretty darn risky with standard parts.Add a heavier piston, and you are really pushing it, and I'm not certain it would hold together very long at those speeds. But it might do it occasionally. Maybe. But it might break the first time you try it, too. That rpm is a risk with the stock rod, and especially if you have a heavy piston in there.

This all is a long way around to get to why I selected 5500rpm as a limit for people who want to retain the stock rod, when doing high performance mods.And why I selected a limit of 6000 rpm for people who want to use the aftermarket Hitchcock's performance crank/rod/bearing package. It is much stronger and can tolerate over 6000 rpms, and tolerate it regularly. But it's expensive. And worth it too.

So, in light of me trying to bring out some speed parts, and reading some of the questions and comments about various combinations, I thought it would be relevant to discuss the "whys and wherefores" about the limitations of engines, and why you can't just do anything with any parts and get away with it. Prudence dictates that if you want your engine to stay together, you must observe the limits of the designs. Or if you want to exceed the limits of the design, you must improve the items which will see additional stress from doing so.

I want you guys to get enjoyment and pleasure out of your performance mods, and not go out and blow up your bottom ends. You can get plenty of fun and performance at 5500 rpm with a Bullet on the street, and if you want do do more, then please get the bottom end parts that will allow it.A word to the wise.

Being familiar with piston speeds, I was surprised with the Ferrari Formula One having a speed of 4890 FPM.

The "dicta" in effect since the recip engine technical heyday with aircraft engines was that the laws of physics precludes any piston speed over 4000 FPM. 4000 FPM was the "wall".

The Formula One motorcycle engineering of the 1960's with wailing five cylinder 250cc Hondas turning 18,000 RPM never touched 4000 FPM piston speeds (very short strokes).

For the average person to conceptulize piston speed and it's effect to the connecting rod - the piston stops dead at BDC and TDC in one 360 degree rotation. The piston accelerates to it's terminal speed velocity in the geometric center of each upward and downward stroke then decelerates to complete stop at TDC and BDC.

Connecting rod alloy composition is a science to itself. Factor in production costs, metal fatigue from lengthwise stretch and compression, side load factors, estimated service life, dimension considerations to name most. And the dynamic stressors in a multitude of planes (directions) are occurring many times per second. The rod cannot be too brittle (hard) or too soft - a fine line of toughness is required.

Any alloy of aluminum has NO stress limits - this means that no matter how small the stress is, it WILL eventually fail under the stresses of intended use.

Any ferrous alloy of the different steels, even soft iron and titanium HAVE stress limits - this means that under a threshold stress limit the component will NEVER fail.

Iron, steels and titanium are engineered for a function under their stress limits and will last forever, shy of an unforeseen outside force by accident.

High grade silicone aluminum alloys will fail in time. They are engineered for a service life, then have to be replaced under routine maintenance.

High grade aluminum alloys even have a shelf life under no stress then have to be replaced. People restoring WWII aircraft know this very well. High grade aluminum Dural bulkheads and stringers turn to baking powder flakes. Japanese WWII aircraft used the best aluminum alloy for wing spars - the few new Zero's captured after WWII left setting around for 50 years and then restored to flying condition - the original wing spars turned to white powder and had to be replaced.

Ace, interesting thread. You had mentioned in the past a targeted RPM range that you want to develop your parts for. Will this range be within this Pomeroy dictum? It seems if a motor is developed to produce peak HP and torque in this range, failures would be almost nonexistent. Maybe a rev limiter would be apart of your engine development. Does the Power Arc have this capability? Keep it coming, can always count on some interesting reading most every time you post.

Ace, interesting thread. You had mentioned in the past a targeted RPM range that you want to develop your parts for. Will this range be within this Pomeroy dictum? It seems if something is developed to produce peak HP and torque in this range, then failures would be almost nonexistent. Maybe a rev limiter would be apart of your engine development. Does the Power Arc have this capability? Keep it coming, can always count on some interesting reading most every time you post.

Blltrdr

No, my expected rev ranges for performance are well above the Pomeroy Dictum level. And also above the stock redline, which is also already above the Pomeroy Dictum level. I suggested 2 rpm performance limits, depending on how the bottom end of the engine is assembled, which I thought was a decent compromise between performance and risk. The final judgment is in the mind of the end-user to determine how much he wants to risk, and how fast he wants to go. I personally bought the Performance Crank/Rod/Bearing assembly from CMW for my engine. Yes, it was expensive, but I took a deep breath and paid the money, because I know why I need it. If you don't get that crank, then you need to be careful how high you rev.

Staying below Pomeroy isn't what performance engines do. And this is why any high performance engine will have the need for improving the overall sturdiness of the moving parts, because they will see heavier duty.To limit peak power rpms to Pomeroy isn't a performance engine. It would be akin to a tractor. But it would probably last for a long time.The key is to push the limits just enough to not break something, and get the power levels that you want. That is what performance modifications are about. If you break something, then you replace it with something stronger. Hopefully it didn't take out the entire crankcase in the process, which is what a broken con-rod will do.See the photo below for what it looks like to throw a rod. It ain't pretty.

Yes, the Power Arc does have a rev-limiter in it, but I don't know what rpm it is set at.My unit is the programmable one, and I can set it myself. Kevin or one of the CMW tech guys would know what the standard Power Arc rev-limiter is set at.

I believe the power-arc ignition can be programmed. I don't know this for fact but I think if one is to check with the manufacturer, they will confirm this. As I said, I think, could be way off on that. H-O racing used to sell Miller aluminum c-rods, maybe still do. I can't recall the specific name for the aluminum but it was claimed to reduce "work hardening" and could be used in wet sump engines with a decent longevity. Ace, great post once again. Perhaps you could espouse on engine blueprinting and prep to improve failure rates.( as in decrease failure rates) As you know, blueprinting is very misunderstood. Toward the end of my involvement with engine building (street/strip) I was just beginning to delve into this subject. It is a lot to know but the increases in durabililty alone seem like they would be worthwhile for those of us with antiquated designs to at least think about.

Just curious if you have had any discussions about this shop and what they offer. I'm having a hard time visualizing the whole process of how they go about beefing up bottom ends. I know you are very good at expressing your knowledge so most of us can get a grip.

Just curious if you have had any discussions about this shop and what they offer. I'm having a hard time visualizing the whole process of how they go about beefing up bottom ends. I know you are very good at expressing your knowledge so most of us can get a grip.

ThanksBlltrdr

Blltrdr,I've seen the Tollgate stuff before, and they even have made some 750cc Bullet singles.I think they are a budget alternative to Hitchcock's stuff.I have not used any of their parts myself, but they seem to get good reports.What they do is bore out the crankpin holes in the flywheels a bit larger, and a bit wider on the flywheels, to increase the stroke and use a larger diameter crankpin to increase the strength, and take up the room where they had to move the holes a little more outboard. This is perfectly fine method to use, and is cheaper because they use the stock flywheels and crankshaft end stubs.They use a new steel rod of the same basic pattern as the stock alloy rod, for strength, and bore the big end a bit larger to take the larger roller bearing on that larger crankpin. Then they re-balance the package and true it.

Hitchcock, Tollgate, and Egli all seemed to think that the stroker is the way to go with a Bullet. And for a low-revving single, it's a good way to get torque and displacement for what the engine is normally used for. As long as you don't want to rev alot of rpms.For the 102mm stroke of the 612, you'd have an rpm limit of 3700 rpm to stay under the Pomeroy Dictum limits. that's over 500 rpm lower than the Pomeroy limits for the 535 or 500. Even with race cams with alot of lift and duration, the 612 peaks at 5500 rpm and peters-out after that. And at 5500 rpm, the 612 has piston speeds of nearly 3700 feet per minute, which is very high, giving tons of stress.I differ with the trend to the longer stroke, and prefer to look at the Manx development, in which the stroke was either kept the same, or even shortened, as development went on, and that was the most successful Brit single ever.I think Tollgate feels it's good to go to the stroker, because he sees a need to beef-up the crank for performance purposes anyway, so he figures that he can just throw in some more stroke along with the new crank and get where he wants to go that way.And that's ok. It works.I think that our stroke is already long, and making it longer just exacerbates the issue, so I'd rather keep the standard stroke and use a good crank and do it that way because piston speeds are lower with the standard stroke than with the stroker.Obviously, it can be done both ways. It just depends on what you want the overall outcome to be. Longer stroke is more "ponderous" and slower to "spin up", but gives alot of "grunt". Shorter stroke is quicker and spins-up faster, and gives higher rev capability.The Bullet is cam-limited as far as lift is concerned, and you have to give it alot of lift to get proper breathing for a 612, which really stresses an already weak valve train.I'm working on improving "area under the curve" to limit lift to what the valve train can take with the 535.I think that I can get equal hp to the 612, out of a 535, and retain the shorter stroke, and give lower stress levels to the crank/rod and the valve train by that method, and have a quicker bike that is less ponderous. And I think that being able to rev to 6000 rpm or maybe even more with the premium crank is key to breaking "the ton".With 18T sprocket, a properly tuned Bullet could do 102mph at 6000 rpms. As long as it has a good power peak up at that rpm, which is what I'm trying to accomplish. Quickness with the "spin-up" for acceleration, and enough rpms to get "the ton" with gearing short enough to not hinder the acceleration.

I suppose it comes down to the modification philosophy of the modifier..And I'd rather do it that way.

I like your philosophy Ace. As long as you stick to your guns, your engine could become the standard in RE performance. I can appreciate the forethought and hard work you have put into your concept. I hope the best for your project and it's final outcome.